The Anatomy of a Transaction

In case this is an output, then this is the address of
the recipient. In case it is an input, then it is the
address of the input which is used to send the tokens
from (i.e. address generated from the private key)

In case there is a spent input,
the signature of the private key is stored here. If no signature is
required, it is empty (all 9's) and can be used for storing the message
value when making a transfer. More to that later.